Most construction machines conventionally have had the engine driving a hydraulic pump that delivers pressure to rotate a hydraulic motor thereby to drive a swing structure serving as an inertial body. In recent years, there have been proposed hybrid construction machines having an engine-driven generator motor capable of generating electric power, an electric storage device such as a capacitor and battery, and an electric motor for driving a swing structure when supplied with electric energy from the electric storage device, whereby fuel economy is improved, noise level is lowered, and the amount of exhaust fumes is reduced (e.g., refer to Patent Literature 1).
Patent Literature 1 discloses a construction machine furnished with control means configured to connect a capacitor and a battery in parallel with the feeders of a swing structure drive electric motor and a generator motor in such a manner that the feeders are supplied with electricity first from the capacitor and then from the generator motor. The construction machine described in this literature allows its capacitor quickly to collect power regenerated from swing braking, thus aiming to improve energy efficiency and reduce the engine size.
The construction machines having the swing structure such as a hydraulic excavator frequently perform combined operations such as a swing boom raising operation in which the boom is raised while the swing structure is being swung. The shift in weight of the bucket as the load on the boom during the combined swing boom raising operation changes the balance between the swing speed of the swing structure and the boom raising speed changes, which can lower the operability for the operator. To reduce such variations in the balance between the swing speed of the swing structure and the boom raising speed, there has been proposed a hydraulic control system for construction machines, the system controlling the capacity of the swing electric motor in such a manner as to distribute engine power appropriately between the swing structure and the boom (e.g., refer to Patent Literature 2).